In a numerically-controlled lathe, a tailstock for supporting a workpiece had in the past often been driven hydraulically, but in recent years, an electrically-driven tailstock driven and controlled by a servo motor has been employed. An electrically-driven tailstock supports a workpiece by pushing, against a center hole of the workpiece, a tailstock center mounted on the tailstock driven by a feed screw coupled to a servo motor. In supporting a workpiece with an electrically-driven tailstock, there exists a problem that, by simply driving and controlling the servo motor to obtain a tailstock support thrust by a drive torque supplied in consideration of the gear ratio of the drive system, a stable support thrust cannot be obtained, due to the transmission efficiency of the transmission mechanism of the drive system. This being the situation, while in a state in which the tailstock is positioned close to the workpiece, the tailstock center is driven at a constant velocity over a preset drive torque measuring distance from a preset position which does not allow contact with the workpiece, and drive torque of the servo motor during that time is detected. A torque value obtained by adding this detected drive torque and a torque command value for the servo motor required for supporting the workpiece is used as a limit value for servo motor drive torque.
The conventional art is now described with reference to FIG. 4. A tailstock support thrust preset in a tailstock thrust setting unit 1 is converted in a torque command converting unit 2 into a torque command value τa for a servo motor 11 while taking into account the gear ratio of the drive system. A support command unit 3 is caused to generate a support command by a manual operation or machining program (not shown), and activates a tailstock control unit 5. The activated tailstock control unit 5 controls torque of the servo motor 11 via a servo control unit 6 and an electric power amplifying unit 10 to thereby drive and control the tailstock, and also performs further positional control using a position feedback supplied by a position detector 12. Here, in the tailstock control unit 5, an average torque value τb, which is an average value of drive torque T detected by a drive torque detecting unit 9 when the tailstock center is moved at a constant velocity V, over a drive torque measuring distance L preset in a tailstock parameter setting unit 4, from a position that is preset in the tailstock parameter setting unit 4 and that is sufficiently spaced away from the workpiece (i.e., a position at which the distance between the tailstock center and the workpiece is greater than or equal to the drive torque measuring distance), is added to the servo motor torque command value τa required for supporting the workpiece, to thereby calculate a torque limit value τc. The calculated torque limit value τc is stored in a torque command value storing unit 7, and supplied to the servo control unit 6. The servo control unit 6 performs torque limiting control for the servo motor 11, in which the torque limit value τc serves as the upper limit.
According to the above-described configuration, the servo motor 11 for driving the tailstock is driven and controlled so that the tailstock center mounted on the tailstock fits into a center hole of the workpiece, and, by stopping the feed of the tailstock when the drive torque T reaches the torque limit value τc output from the servo control unit 6, the workpiece is supported by a predetermined support thrust.
According to the above-described conventional art, in a zone over the drive torque measuring distance from the preset position sufficiently spaced away from the workpiece, drive torque in an unloaded state is detected, which corresponds to a torque value that results in a drop in thrust due to the transmission efficiency of the transmission mechanism of the drive system. For this reason, in this zone, it is not possible to support the workpiece. For cases such as those in which the workpiece is held by hand when causing the tailstock center to provide support, it is desired to be able to start drive of the tailstock from a position closer to the workpiece. However, the preset drive torque measuring distance is a uniform distance for all cases regardless of the servo characteristics, and is set at a distance longer than that actually required for the drive torque measurement, so that, disadvantageously, the zone in which a workpiece cannot be supported is excessively large.